Stated plainly: 5 mg · a GLP-1 receptor agonist.
The units are where I keep going wrong, so please be explicit about them.
I have sanity-checked the order of magnitude and it seems right, which is not the same as being right.
Where is my error, and what is the correct working?
On a 5 mg vial every percentage point of water and counter-ion is 0.05 mg, so the answer is a multiplication once you have the net peptide content off the certificate. Settle first whether the 5 mg on the label is gross fill or net peptide, because the two differ by exactly the quantity being asked about. If the certificate quotes net peptide content of 85 per cent, a 5 mg gross fill holds 4.25 mg of peptide; at 78 per cent it holds 3.9 mg. That 0.35 mg gap is 7 per cent of the label, larger than any purity difference anybody argues about, and it is invisible to a purity figure because water and acetate are not impurity peaks. Amino acid analysis or a nitrogen determination gives you the number; an HPLC area per cent never will.
The relevant detail is that two measures of the same vial can agree on purity and disagree on content by a few per cent, which usually means the content assay used a different standard.
Water content and counter-ion content are part of the gross mass but not part of the content assay result, which is why the two do not sum to label claim.
A content assay is always paired with a purity assay because purity tells you what fraction of the measured mass is the target and content tells you the total measured mass.
The limitation is that a quantitative method is only as good as the standard it uses, and a cheap standard is a false economy.
Ask for both the purity and the content, and do not accept purity alone.
HPLC purity, identity confirmation and quantified content on the vial you actually hold. Reports arrive with the chromatogram attached, not just a number.
Submit a sampleFounded 1998. ISO 9001 and cGMP certified, 1,500+ staff and 200+ patents. The synthesis house behind a great many of the vials that get sent out for testing - batch-specific documentation with every order.
Visit GL BiochemMechanically, most research-grade certificates report purity without content, which is exactly backwards from what users actually need.
The single most common reason for disagreement between a supplier content figure and an independent assay is using different standards.
To be exact about it, running multiple independent aliquots of the same sample should give results that agree to within the method precision, which is usually one to three per cent.
The caveat is that content assay costs more than purity, so most people do not do it, which is exactly why it is valuable on the first lot from a new supplier.
If a supplier gives you content without the standard's purity, ask them to provide it.
In practice, reading a content result requires knowing the purity of the standard against which the sample was quantified, because your result inherits that uncertainty.
Quantitation against a certified reference material assumes the sample and standard are treated identically through the analytical method, which is why the method for calibration matters as much as the method for measurement.
The underlying point is that for peptides at 214 nanometres the response is roughly proportional to the number of peptide bonds, so truncation impurities have lower response factors and overestimate content.
Where content data have been published from testing services on common peptides, the spread between services on identical material is typically a few per cent.
The practical summary: if you are ordering from a new supplier, budget for content assay on the first lot.
Ask PeptideStack is a static archive. Posting is closed, but the norms are worth stating: answer the question that was asked, show your working, cite the trial or the certificate, and say plainly where the evidence runs out.